Why is calibration accuracy important?
Most manufacturers use a process known as ‘wet calibration’ to test flow meters under good flow conditions on a flow rig. This process is critical in ensuring the flow meter delivers the best possible accuracy in everyday use. The resulting data is then used by the manufacturer to show that the flow meter has been calibrated for accurate measurement within the limits of its specification.

In the majority of applications, where flow meters are used to give an indication of the rate at which a liquid or gas is moving through a pipeline, high accuracy is not a crucial factor. However, for processes such as in the food processing or chemical industries, flow meter accuracy is often the deciding factor between optimum product quality and wasted product potentially running into thousands of pounds. The problem is that no two manufacturers’ flow rigs are exactly the same. Consequently, it is impossible for them to guarantee to customers that their flow meters are calibrated under exactly the same conditions.

To be sure that your flow meter has been calibrated as accurately as possible, you should be sure to ask manufacturers the following questions:

Does the calibration figure include flow rig uncertainty?
Flow rig uncertainty takes into account all factors that could affect the meter’s flow measurement accuracy during calibration. Sources of uncertainty can include variations in testing times, valve response, and the effects of obstacles in the line or variations in the properties of the fluid being measured.

In reality, manufacturers tend to base their calibration uncertainty figures on the uncertainty of their reference device. These devices provide a known value against which to compare measurement performance and can include methods such as a weigh tank, volume tank or meter prover. Any deviations from this value are then used to calculate the uncertainty value.

The resulting uncertainty value, however , is the uncertainty of the primary device only, not the uncertainty of the whole calibration process. The reputable manufacturers will use this figure as the base for their uncertainty calculations; the less scrupulous state it as the calibration uncertainty itself.

How was the uncertainty value calculated?
Various standards exist which define procedures for measuring and expressing uncertainty. ‘Uncertainty and Confidence in Measurement’ Document M3003 published by the UK Accreditation Service (UKAS) sets out procedures for accredited laboratories for estimating uncertainty during calibration. This publication defines in detail the complex calculation that is necessary to arrive at the real uncertainty of a calibration facility. If properly adhered to, it should allow comparisons of calibration laboratories’ uncertainties.

Even this publication, however, does not account for factors such as the dynamics of the rig or its operating procedures. The situation is further complicated by the response of flow to a change in conditions.

It is well known that if a step change occurs in a ‘stable’ flow regime, then it can take up to five minutes for the flow to again reach a stable condition.

Most flow rigs cannot achieve a step change, needing a finite time for valves in the line to fully open and close, for example.

ISO standards 9368 and 4185 set out guidelines for uncertainty in the measurement of time during calibration, with a recommended period of at least 30 seconds. Most flow calibration experts will agree that a calibration time of at least 60 seconds is required. Despite this, some manufacturers use times that are only 25% of this figure, introducing still more uncertainty into the calibration.

It is for these reasons, amongst others, that it is unlikely for any manufacturer to be able to achieve calibration accuracy significantly better than 0.1%. Those who claim that their rig is as accurate as their primary device are simply misleading their customers.

Asking the simple questions suggested here will enable you to make a more informed choice about which manufacturers can truly offer you the most accurate flow meter for your needs.